This invention relates to oil well pumping and especially to pumping with flexible pumping strand and more particularly to flexible pumping strand assemblies equipped with safety devices designed to prevent ballooning and rupture of the strand due to events or accidents which may produce excess pressure in the strand.
U.S. Pat. No. 3,637,341 issued Jan. 25, 1972 to Horton et al. and assigned to the assignee of the present invention discloses a corrosion protection system for flexible pumping strand used in oil wells. In accordance with the invention disclosed in said patent a flexible pumping strand is provided with a flexible outer jacket customarily formed of a plastic resin composition, which jacket isolates the wires of the strand from the surrounding environment and also encloses a corrosion inhibiting fluid within the strand. In accordance with the preferred operation of this prior invention to corrosion inhibiting fluid is passed slowly through the strand either continuously or intermittently so that the inhibitor is continuously or periodically renewed in order to overcome the inhibitor exhausting effects of any corrosive substances which may gain access to the interior of the pumping strand through the strand jacket. Such corrosion including substances may gain access to the interior of the strand either through a slight permeability of the jacket over long periods of time or through defects in the jacket. In accordance with the prior invention the corrosion inhibiting fluid is selected or adjusted to have a specific gravity substantially similar to the specific gravity of the liquid components or major liquid components of the surrounding environment in the oil well so that the pressure due to the head of corrosion inhibiting liquid in the interior of a long length of strand extending down an oil well will be similar to the pressure in the oil well outside of the flexible pumping strand. If the pressure differential between the interior of the strand and the surrounding environment on the exterior of the strand is greater than the ballooning or yield strength of the plastic of the strand jacket the jacket will balloon and eventually rupture usually at the location of the greatest pressure differential, particularly if the exterior pressure is less than the interior pressure. If the location of the pressure differential is at the bottom of the strand due to the use of too dense a corrosion inhibiting liquid the ballooning of the jacket will occur near the bottom of the well or the lower end of the strand, while if the corrosion inhibiting liquid is not dense or heavy enough it will have to be forcibly pumped to the lower portions of the strand and the force of the pump may cause ballooning or rupture of the strand jacket near the top of the well or in the upper portions of the flexible pumping strand.
It has been discovered that while the provision of a corrosion inhibiting liquid in the interior of a flexible pumping strand having a specific gravity similar to the specific gravity of the surrounding environment in accordance with the previous invention is effective to prevent ballooning, rupture or other damage to the jacket of the strand during normal operation or use of the strand to operate a down-hole pumping apparatus, difficulty with ballooning and rupture of the strand jacket may still occur if for some reason the well tubing becomes suddenly or even in some cases slowly filled with gas so that the density and weight of the environment at the exterior of the strand becomes much less than the density or weight of the corrosion inhibiting liquid in the interior of the flexible pumping strand. In such case if ballooning and rupture of the strand jacket is to be avoided the pressure of the liquid in the interior of the strand must be relieved before the ballooning strength of the plastic jacket is exceeded by the pressure differential between the interior and the exterior of the strand.
Loss of exterior pressure in an oil well may occur in a number of manners of which the following are notable examples.
a. The pumping apparatus at the bottom of the well may fail so that oil is no longer being pumped. Normally this will involve in the usual type of pump the failure of both the lower ball valves to close. If the valves fail to close properly the oil in the well tubing will drain out through the pump and be replaced with gas or vapor either from the well or from the atmosphere.
b. The well tubing may develop a hole or a series of perforations through which the oil in the tubing may drain. Holes or openings in the tubing may originate as a result of corrosion or abrasion through the tubing, physical puncture of the tubing or because the tubing unscrews at a coupling. Accidental unscrewing of tubing couplings is by no means uncommon in many oil wells.
c. The oil well pump may unseat from its seating arrangement in the tubing at the bottom of the well. Such unseating of the pump may often occur due to binding of the plunger rod against the packing in the pump. Binding of the plunger may be due to loose scale or sand getting in the packing of the pump or to corrosion of the pumping rod, dropping of objects into the well and the like. If the plunger rod binds in the packing the entire pump may be lifted during the upstroke of the pump and the pump unseated in the well tubing, whereupon the oil in the tubing will drain from the tubing. Very often when the pump fails due to various types of breakage or the like the pump body will also become unseated within the well tubing allowing the oil to drain from the tubing.
d. A bubble or pocket of gas from the surrounding strata may gain access to the well and expel all the oil within the tubing leaving only a light gas. This eventuality is liable to occur in wells which do not have enough gas pressure to cause the oil to flow from the well by gas pressure alone without pumping, but do have some residual gas pressure in the surrounding strata. Sometimes the entrance of gas into the well is very sudden causing a very rapid change in pressure in the well.
All of the foregoing eventualities which result in the substitution of a gaseous medium for a liquid medium in the well, usually unexpectedly and quite often very rapidly, may cause a significant pressure differential between the interior of the pumping strand and the exterior environment which, if the flexible pumping strand is of any great length, will often quickly exceed the ballooning or rupture strength of the plastic jacket of the pumping strand. Exceeding the ballooning strength of the plastic jacket quickly results in ballooning of the jacket which, if continued quickly leads to rupture of the strand jacket allowing access of the corrosive exterior environment to the metal wires of the interior of the strand with resultant corrosion and also allowing the corrosion inhibitor in the interior of the strand to escape with the same ultimate effect. Even if the rupture strength of the strand jacket is not exceeded and the jacket only balloons, the jacket will be weakened and its thickness decreased and it may be exposed to physical damage during operation or removal from the well. Some plastic materials, furthermore, may exhibit rupture under excess pressure without initial ballooning and other plastics may exhibit either ballooning and then rupture or rupture without initial ballooning depending upon the environment or conditions. The foregoing list of eventualities which may result in the loss of oil from the well tubing is exemplary only and it will be understood that it is neither exhaustive nor exclusive.